#13125: GHC Panics when encoding Russel's paradox using GADTs -------------------------------------+------------------------------------- Reporter: tysonzero | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: duplicate | Keywords: panic, GADTs Operating System: MacOS X | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by carter): * status: new => closed * resolution: => duplicate Comment: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html #bugs-in-ghc see the remark at the end {{{ GHC’s inliner can be persuaded into non-termination using the standard way to encode recursion via a data type: data U = MkU (U -> Bool) russel :: U -> Bool russel u@(MkU p) = not $ p u x :: Bool x = russel (MkU russel) The non-termination is reported like this: ghc: panic! (the 'impossible' happened) (GHC version 7.10.1 for x86_64-unknown-linux): Simplifier ticks exhausted When trying UnfoldingDone x_alB To increase the limit, use -fsimpl-tick-factor=N (default 100) with the panic being reported no matter how high a -fsimpl-tick-factor you supply. We have never found another class of programs, other than this contrived one, that makes GHC diverge, and fixing the problem would impose an extra overhead on every compilation. So the bug remains un-fixed. There is more background in Secrets of the GHC inliner. }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13125#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler